Method for purifying steam



Feb. 28, 1939. B BLQCK r AL 2,149,160

METHOD FOR PURIFYING STEAM Filed Aug. 25, 1938 Fee wqier Patented Feb.28, 1939 UNITED STATES PATENT OFFICE METHOD FOR PURIFYING STEAMApplication August 25, 1938, Serial No. 226,666

1 Claim.

In the operation of high pressure steam plants a difficulty has beenfound resulting from the fact that materials, particularly salts,contained in the boiler water are carried along by the steam as it isgenerated, such materials getting into the superheater, the steam lineand the turbines and there form deposits. As is well known, when steamis generated, water in finely divided form is carried along. Thismoisture content of the steam contains solids in the form of minuteparticles held in suspension or in solution. These solids are removedfrom the steam by the removal of the moisture by means of a steamseparator. In addition, however, there are in the steam other materialswhich assume a gaseous form in the steam boiler as the steam isgenerated. These materials, which are mainly salts, flow in the form ofsalt vapor or superheated gas to the prime mover together with thesuperheated steam and cannot be removed by a steam separa-tor.

The operating steam for driving the prime mover is in other words, notpure water vapor but is a mixture of water vapor, and salt vapor orsublimates. The salts (sublimates)- can be condensed and changed fromtheir gaseous state to solid state by suitably cooling them. Suchcooling to the point where they segregate out in solid form occursordinarily during the expansion of the steam in the turbine. During theflow of the steam through the blades and nozzles the salts are thereforedeposited on these parts thereby cutting down the free cross areabetween blades and in the nozzles and interfering with a the operationof the turbine.

Up to the present time operators have been forced when the eificiency ofa turbine began to diminish from this cause, to take the turbine out ofoperation and to wash it out with clean distilled water, removing thesalts (sublimates) which had become deposited.

Such a procedure is possible only where the turbine can be shut down atfairly short intervals. In many cases, e. g. in the case of powerplants, however, turbines which are in service must be kept on the linefrom 6,000 to 8,000 hours without interruption.

Different means have been tried to remove these objectionable salts. Ithas been suggested, for example to send the saturated steam coming fromthe boiler through a steam washing device where such steam is washedwith feed water on its way to the boiler which is substantially free ofsalts. Again steam separators have been used in connection with theboilers to reduce the after it has been generated and superheated.

For this purpose 'a nozzle or diffuser was provided forming a means forexpanding and again compressing it, a separating device such as a groovewith a collecting channel being provided between these two operations tocatch and carry away the moisture and impurities separated out. Thepresent invention also proposes to purify the high pressure steam bycooling it. In accordance with it, however, a non-contact heat exchangeris used in which the steam is cooled down so far by means of coolingsurfaces that salts are deposited on the cooling surfaces. Thetemperature of these cooling surfaces is kept below that of saturatedsteam of the same pressure as the superheated high pressure steam beingpurified. The steam from which the salts have been removed is againsuperheated in a second superheater.

The salts in such a device are deposited in a form similar to icecrystals deposited in winter on free objects. For this purpose thevelocity of fiow must be relatively small, to allow the formation of thedeposits of the salt particles (sublimates). Such action is easy toobtain in a surface heat exchanger in which the superheated steam iscooled, the velocities in such a device being low. In addition, such asurface heat interchanger readily accommodates means whereby salts whichare deposited can be caught and removed.

In practicing the invention a suitable cooling medium for the heatexchanger is the interstage steam on its way from one expansion stage ofthe turbine to a lower pressure stage. The temperature of this steamwill definitely be below the saturation temperature of the high pressuresteam to be purified.

Another cooling medium that may be used is feedwater on its way to theboiler. In this case the water from the economizer is sent to the heatexchanger at a temperature also lower than the saturated temperature ofthe operating steam.

It is not necessary that all of the steam itself should be cooled downto the temperature at which the gaseous salts become deposited assolids. This deposit will occur even if the steam remains of highertemperature provided the gaseous salts (sublimates) come into contactwith the cooled surfaces.

Fig. 1 is a diagrammatic View of a steam plant embodying the presentinvention, and

Fig. 2 is a diagrammatic view illustrating a modification of part of thesystem.

A simple form of apparatus to carry out the invention is shown in theaccompanying drawing. Here a is a high pressure boiler. The steamgenerated in this boiler flows through the pipe I) to the firstsuperheater c in which the entrained water and salts are completelyvaporized. The mixture of superheated steam and sublimate is then led tothe heat interchanger d in which the superheated steam is cooled bycoming into contact with the surfaces i which are kept cool by means ofinterstage steam from the turbine g. The purified steam is taken fromthe heat interchanger and carried to the secondary superheater e whereit is brought up again to the desired superheat temperature for use inthe high pressure stage of the two-stage turbine.

The exhaust steam from the high pressure stage is conveyed through pipeh to cool the surfaces of the heat exchanger and is then carried throughthe pipe is to the second stage Z. The exhaust from this second stage isthen carried to the condenser 111..

As illustrated in Fig. 2 the feed water on its way to the boiler isemployed as the cooling medium in the heat interchanger d. The feedwater forced through line 11 and heated in the economizer s of theboiler passes through the coils t of the heat interchanger and is thendirected to the boiler through the line it.

The salt (sublimate) deposited on the cooled surfaces of the heatexchanger is removed at intervals preferably by permitting saturatedsteam to condense on the cooling surfaces the resulting condensatedissolving and washing off the deposited salts. The dissolved salts canthen be removed from the heat exchanger through the drain n.

What we claim is:

The process of purifying superheated high pressure steam containingsalts in gaseous form coming from a boiler comprising the steps ofcausing the steam to fiow in contact with one side of a wall of heatconductive material, cooling the wall by causing a fluid to flow incontact with the other side of the wall of a temperature and in quantitysufficient to keep said wall at a temperature below that of saturatedsteam of the same pressure as the superheated steam, whereby the saltsare deposited in solid form on said first side of the wall, andperiodically washing said deposits ofi, the said periodic washing beingeffected by bringing saturated steam from the boiler into contact withthe first side of the wall and permitting it to condense to form thewater to wash oil the salts.

BERTHOLD BLOCK. OTTO HARTMANN.

